"ツ" aka "Tsu" was adopted as the official currency symbol of the project.
Bad idea IMO since ツ is so common in Japanese. It'd be like using the letter "G" as their symbol.
Yes, it may be odd particularly for Japanese, but ツ can be used for counting almost everything.
The following site is good to grab the meaning of ツ.
https://www.tofugu.com/japanese/japanese-counter-tsu/"The Japanese counter 〜つ is probably the most useful counter you can learn. It just counts
"things." And when we say "things," we really mean it: it can count literally anything. That's pretty broad, though, so let's break it down into various categories."
That's why Japanese Grin supporters haven't denied ツ to be used as Grin's symbol as far as I know.